Switching circuit

ABSTRACT

The present invention provides a switching circuit comprising a current detecting circuit that detects current waveform of a main switch in real time waveform and outputs it in low impedance. The switching circuit comprises the current detecting circuit having the main switch ( 11 ), of which on-voltage shows resistance characteristics, wherein a gate terminal is connected to a driving circuit ( 4 ) and further either terminal of drain or source thereof is connected to a fixed potential ( 3 ) and the other terminal thereof is connected to a load circuit ( 2 ) respectively, a first resistance element ( 21 ), of which resistance value is higher than on-resistance of the main switch connected to the fixed potential ( 3 ), a subsidiary switch ( 12 ), a source terminal thereof is connected to the first resistance element ( 21 ), an amplifier ( 6 ) that amplifies comparatively voltage generated in the first resistance element ( 21 ) and outputs to a gate terminal of the subsidiary switch ( 12 ) and the second resistance element ( 22 ) that generates voltage by amplifying on-current of the main switch ( 11 ) by connecting to the drain terminal of the subsidiary switch ( 11 ).

FIELD OF THE INVENTION

The present invention relates to a circuit for detecting current of aswitch by detecting on-voltage of a power switch.

RELATED ART

A first conventional example of a switching circuit having a currentdetecting circuit is shown in FIG. 10. This conventional example isequipped with a main switch 111 composed of MOSFET, of which on-voltageshows resistance characteristics, and a gate terminal of the main switch111 is connected to a driving circuit 104. And a drain terminal of themain switch 111 is connected to a fixed potential 103, and a sourceterminal thereof is connected to a load circuit 102. A current detectingcircuit installed in the switching circuit has a current detectingswitch 114 wherein a drain terminal thereof is connected to the fixedpotential and a gate terminal thereof is connected to the drivingcircuit 104. Further, a source terminal of the current detecting switch114 is connected to a resistance element 121 and a positive-terminal ofan amplifier 106 and the source terminal of the main switch 111 isconnected to a negative-terminal of the amplifier 106, which isconstructed so that current can be detected by amplifying voltage of aresistance element 121.

And a second conventional example is shown in FIG. 11, which isdescribed based on Japanese Patent No. H1-26250. This conventionalexample comprises the main switch 111 composed of MOSFET, of whichon-voltage shows resistance characteristics, the gate terminal thereofis connected to the driving circuit 104. And the drain terminal of themain switch 111 is connected to the fixed potential 103 and its sourceterminal is connected to the load circuit 102. The current detectingcircuit installed in this switching circuit comprises the currentdetecting switch 114, whose drain terminal is connected to the fixedpotential and whose gate terminal is connected to the driving circuit104. And further by applying a standard current to the current detectingswitch 114, on-voltage of the current detecting switch 114 andon-voltage of the main switch 111 are to be amplified comparatively bythe amplifier 106.

At first, in the first conventional example, in order that current ofthe resistance element 121 is proportional to on-current of the mainswitch 111, voltage of the resistance element 121 must be half of theon-voltage of the current detecting switch 114 or less. Therefore, inorder to amplify waveform up to the level usable for a control circuit,an amplifier needs to have high gain capacity. Because current waveformincludes components of high frequency, it was actually difficult toamplify the current wave up to high frequency keeping a high gaincapacity.

And in the second conventional example, because by applying a standardcurrent to the current detecting switch 114, on-voltage of the currentdetecting switch 114 and on-voltage of the main switch 111 is to beamplified comparatively, it has been a problem that an output signal isnot real time current waveforms so that results of comparison are aboveor below a defined current level.

DISCLOSURE OF THE INVENTION

In view of the above problem, the present invention is to provide aswitching circuit with a low cost having a current detecting circuitthat detects current waveform of the main switch in form of a real timewaveform in high speed and outputs it in form of a large signal with lowimpedance.

In the present invention, the above-described object is achieved byproviding the current detecting circuit that can output large currentwaveform while keeping difference of input voltage of an amplifier to beabout zero volt. And by connecting the third switch, of which gatesignal synchronizes that of the main switch, between a first resistanceelement and a subsidiary switch, voltage waveform of the secondresistance element becomes similar to that of current waveform of themain switch, so that the current waveform with a high speed can betreated as a large voltage signal.

When the first resistance element made from poly-silicon resistance isformed on an insulating film, insulation between the main switch and thefirst resistance element can be formed easily and a highly integratedcircuit may be possible and parasitic capacity etc. becomes small sothat high speed operation becomes possible. When the first resistanceelement is formed so as to have its temperature coefficient with higherpositive value than that of the main switch, in the current detectingcircuit using the first resistance element, it is possible to easilyprovide a switching circuit with stronger function ofanti-heat-bursting. Therefore, a current detecting circuit with manyexcellent features can be easily formed.

That is, to solve the above-described problems, the present inventionhas structures set forth below.

According to a first embodiment of the present invention, a switchingcircuit having a switching element, comprises a current detectingcircuit having a main switch composed of MOSFET, whose on-voltage showsresistance characteristics, wherein a gate terminal thereof is connectedto a driving circuit, and further one of a drain terminal and a sourceterminal thereof is connected to a fixed potential and the otherterminal is connected a load circuit, a first resistance element havinghigher resistance value than on-resistance of the main switch, asubsidiary switch composed of MOSFET, the source terminal of which isconnected to the first resistance element, an amplifier that amplifiescomparatively voltage generated in the first resistance element andon-voltage of the main switch and outputs it to a gate terminal of thesubsidiary switch, and a second resistance element that generatesvoltage by amplifying on-current of the main switch by connecting to adrain terminal of the subsidiary switch.

According to a second embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, one of terminals of the first resistance element is furtherconnected to a fixed potential as in the main switch.

According to a third embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, a third switch, whose gate signal synchronizes that of themain switch, is connected between the first resistance element and thesubsidiary switch.

According to a fourth embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, one of terminals of the first resistance element is connectedto a fixed potential as in the main switch and the third switch, whosegate signal synchronizes that of the main switch is connected betweenthe first resistance element and the subsidiary switch.

According to a fifth embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, one of terminals of the first resistance element is connectedto a terminal which is connected to a load circuit of the main switch,and the other terminal of the first resistance element is connected tothe source terminal of the subsidiary switch.

According to a sixth embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, the first resistance element has the same cell structure asthe main switch and is MOSFET biased by a fixed gate or poly-siliconresistance.

According to a seventh embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, the first resistance element has the same cell structure asthe main switch and is MOSFET biased by fixed gate or poly-siliconresistance, wherein one of terminals of the first resistance element isconnected to a fixed potential as in the main switch.

According to an eighth embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, the first resistance element has the same cell structure asthe main switch and is MOSFET biased by a fixed gate or poly-siliconresistance wherein the third switch, whose gate signal synchronizes thatof the main switch, is connected between this first resistance elementand the subsidiary switch.

According to a ninth embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, the first resistance element has the same cell structure asthe main switch and is MOSFET biased by a fixed gate or poly-siliconresistance, wherein one of terminals of the first resistance element isconnected to a fixed potential as in the main switch and the thirdswitch, of which gate signal synchronizes that of the main switch, isconnected between this first resistance element and the subsidiaryswitch.

According to a tenth embodiment of the present invention, in theswitching circuit according to the first embodiment of the presentinvention, the first resistance element has the same cell structure asthe main switch and is MOSFET biased by the fixed gate or poly-siliconresistance wherein one of terminals of this first resistance element isconnected to a terminal which is connected to the load circuit of themain switch and the other terminal of the first resistance element isconnected to the source terminal of the subsidiary switch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing outline of a first embodiment of aswitching circuit according to the present invention.

FIG. 2 is a circuit diagram showing outline of a second embodimentthereof.

FIG. 3 is a circuit diagram showing outline of a third embodimentthereof.

FIG. 4 is a diagram of operation waveform of the third embodiment shownin FIG. 3.

FIG. 5 is a circuit diagram showing outline of a fourth embodimentthereof.

FIG. 6 is a circuit diagram showing outline of a fifth embodimentthereof.

FIG. 7 is a circuit diagram showing outline of a sixth embodimentthereof.

FIG. 8 is a circuit diagram showing outline of a seventh embodimentthereof.

FIG. 9 is a circuit diagram showing outline of an eighth embodimentthereof.

FIG. 10 is a circuit diagram showing outline of a first conventionalexample of a switching circuit.

FIG. 11 is a circuit diagram showing outline of a second conventionalexample.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a switching circuit according to the present inventionwill be described hereafter with reference to attached figures.

Embodiment 1

The first embodiment of the switching circuit according to the presentinvention is shown in FIG. 1. 1 is an input power source, 2 is a load, 3is fixed potential, 4 is a driving circuit, 6 is an amplifier, 11 is amain switch, 12 is a subsidiary switch, 13 is a third switch, 14 is acurrent detecting switch, 15 is a high side switch, 21 is a firstresistance element, and 22 is a second resistance element.

The switching circuit according to the present embodiment comprises amain switch 11 composed of p-channel MOSFET, whose on-voltage showsresistance characteristics wherein a gate terminal of the main switch 11is connected to a driving circuit 4. And a source terminal of the mainswitch is connected to the fixed potential 3 and a drain terminalthereof is connected to the load circuit 2. In addition, the fixedpotential 3 may be connected to the drain terminal of the main switch 11and the load circuit 2 may be connected to the source terminal of themain switch 11. This switching circuit comprises the current detectingcircuit, which comprises the first resistance element 21 having higherresistance value (10000 times for example) than on-resistance of themain switch 11, and the first resistance element 21 is connected to thefixed potential 3 or the load circuit 2. Moreover, the first resistanceelement 21 may be an ordinary diffusion resistance or a poly-siliconresistance. And by setting temperature coefficient of the firstresistance element 21 to the same value as that of the on-resistance ofthe main switch 11, an accurate current detecting circuit can beconstructed. And the switching circuit comprising this current detectingcircuit can be set up so as to shift a detecting point of over currentto a lower current side according to elevation of temperature.

The current detecting circuit comprises the subsidiary switch 12composed of p-channel MOSFET, wherein the first resistance element 21 isconnected to a source terminal thereof. And the current detectingcircuit comprises the amplifier 6. The positive-terminal of theamplifier 6 is connected to the drain terminal of the main switch 11,and a negative-terminal thereof is connected to the source terminal ofthe subsidiary switch and the first resistance element 21, and an outputthereof is connected to a gate terminal of the subsidiary switch 12. Theabove-described constructions aims to output to the gate terminal of thesubsidiary switch 12 by amplifying comparatively voltage generated atthe first resistance element 21 and on-voltage of the main switch 11.And further the current detecting circuit comprises the secondresistance element 22 that is connected to the drain terminal of thesubsidiary switch 12 and is constructed so as to generate voltage byamplifying on-current of the main switch 11.

The switching circuit with the current detecting circuit is constructedas described above and operated as describing thereafter. When the mainswitch 11 is turned on, the main switch 11 generates on-voltage, whichis detected by the amplifier 6. And by turning on the main switch 11,voltage is generated in the first resistance element 21 that isconnected to the fixed potential 3 and detected by the amplifier 6.On-voltage detected by the amplifier 6 and voltage generated in thefirst resistance element 21 is amplified comparatively so as to generateand output voltage to the gate terminal of the subsidiary switch 12.That is, since potential difference of the input terminal of theamplifier 6 is about zero volt when on-current is detected, the currentdetecting circuit composed of the two resistance elements 21, 22, thesubsidiary switch element 12 and the amplifier 6 can easily obtain highgain and amplify up to a high speed signal and supply a current signalof on-current of the main switch 11 with low impedance output.

Embodiment 2

The second embodiment is shown in FIG. 2. This embodiment is almost thesame as that of the first embodiment. This embodiment is characterizedin that the main switch 11 is composed of n-channel MOSFET. Detection ofcurrent can be also conducted by the same way as that of the firstembodiment. However, polarity characteristics of output voltage of thedriving circuit 4 is different from that of the first embodiment, thatis, in the first embodiment, the main switch 11 becomes conductive bynegative gate potential to the source potential, on the other hand, themain switch 11 becomes conductive by positive gate potential to thesource potential in the case of this embodiment.

Embodiment 3

The third embodiment is shown in FIG. 3. In this embodiment, the firstresistance element 21 is connected to a source terminal of the thirdswitch 13, wherein a drain terminal is connected to the source terminalof the subsidiary switch 12. And the gate terminal of the third switch13 is connected to the driving circuit 4 and the gate signal of thethird switch 13 is so constructed as to synchronize with that of thegate signal of the main switch 11. That is, since the main switch 11 isp-channel MOSFET in this embodiment, the third switch 13 is alsop-channel MOSFET.

Because in the case of the third embodiment, the gate signal of the mainswitch 11 is so constructed as to synchronize with that of the gatesignal of the third switch 13, when the main switch 11 is on off-state,and the third switch 13 is also on off-state, it is possible to preventlarge current from flowing through the first resistance element 21.Therefore, a current detecting circuit with electric power of a lowconsumption is realized. And further operation waveform in this thirdembodiment is shown in the next FIG. 4.

FIG. 4 shows, from the top, voltage Vgs between the gate and the sourceof the main switch 11, voltage Vds between the drain and the source ofthe main switch 11, current Ids between the drain and the source of themain switch 11 and current signal output generated in the secondresistance element 22. In the case of the third embodiment, since themain switch 11 is p-channel MOSFET, Vgs, Vds and Ids functions bynegative voltage and negative current. It is assumed as the case ofcomponents of parasitic capacity contained in load that a peak currentIdsp is generated at the time of turn-on of the main switch 11. Voltageof the second resistance element 22 is similar to inversed pole ofcurrent of the main switch 11. And in FIG. 3, ton, and toff representsdelay time of at the time of turn-on, turn-off respectively, and t1, andt2 represents drop time and rising edge time, respectively.

And further, the main switch may be n-channel MOSFET also in thisembodiment. But since the gate signal of the third switch 13 is soconstructed as to synchronize with that of the main switch 11, the thirdswitch 13 is constructed by n-channel MOSFET in many instances. In thiscase, when pole characteristics of output voltage of the driving circuit4 is positive potential to the source potential, the main switch 11 isoperated.

Embodiment 4

The fourth embodiment is shown in FIG. 5. In this embodiment, inaddition to the structure of the third embodiment, a delay circuit 5 isconnected between the gate terminal of the third switch 13 and thedriving circuit 4. And in this embodiment, the first resistance elementhas the same cell structure as the main switch 11 and uses the currentdetecting switch 14 constructed by MOSFET biased by a fixed gate. Andfurther this current detecting switch 14 has higher on-resistance value(10000 times for example) than on-resistance of the main switch 11. Thegate terminal of the current detecting switch 14 is connected to thecircuit 7 for generating a low-level gate voltage.

In this embodiment, by connecting the delay circuit 5 between the gateterminal of the third switch and the driving circuit 4, when the mainswitch 11 of high-side is turned on, it is possible to prevent parasiticcomponent of capacity charging current Idsp in the load circuit 2 fromappearing as a detecting signal and it is also possible to preventmalfunction of an over current protecting circuit. And in this case, adelay time is usually about t1. Therefore, the delay circuit 5 causesthe delay time by about t1, only when the third switch 13 is turned on,and the delay time is zero, when the third switch 13 is turned off.

And since the first resistance element has the same cell structure asthat of the main switch 11 and has a higher resistance value than thatof on-resistance of the main switch 11, and the current detecting switch14 biased by the fixed gate is used, the on-resistance of this currentdetecting switch 14 has the same temperature characteristics andvariation as these of the main switch 11, so that accuracy of detectingcurrent can be easily improved. Therefore a current detecting circuitwith a high reliability can be easily realized.

Embodiment 5

The fifth embodiment is shown in FIG. 6. This embodiment shows a currentdetecting circuit of reversed direction current. In the currentdetecting circuit shown in the first to the fourth embodiments it isassumed that current, flowing in the direction from the power 3 throughthe main switch 11 to the load circuit 2, is positive. In the fifthembodiment, a current detecting circuit is provided, wherein current,flowing in the direction from the load circuit 2 through the main switch11 to the input power 1, is positive. The amplifier 6 makes thesubsidiary switch 12 operate, only when a positive potential isimpressed between the source and drain of the main switch 11. At thattime, an inversed current waveform of the main switch 11 is generated inthe second resistance element 22, and the inversed current can bedetected by using voltage waveform of the second resistance element 22.

Embodiment 6

The sixth embodiment is shown in FIG. 7. In the fifth embodiment, thefixed potential 3 is a negative pole of input voltage. This switchingcircuit comprises the main switch 11 composed of n-channel MOSFET, whoseon-voltage shows resistance characteristics, wherein the gate terminalthe main switch 11 is connected to the driving circuit 4. And the sourceterminal of the main switch 11 is earthed and the drain terminal thereofis connected to the load circuit 2. In addition, in the case that themain switch 11 is p-channel MOSFET, the drain terminal of the mainswitch 11 may be grounded and the load circuit 2 may be connected to thesource terminal of the main switch 11. This switching circuit comprisesthe current detecting circuit, which comprises the first resistanceelement 21 having higher resistance value (10000 times for example) thanthat of on-resistance of the main switch 11, and the first resistanceelement 21 is grounded. Further, the first resistance element 21 may bea conventional diffusion resistance or preferably a poly-siliconresistance. And by setting temperature coefficient of the firstresistance element 21 the same as that of on-resistance of the mainswitch 11, an accurate current detecting circuit can be constructed. Andthe switching circuit using this current detecting circuit can be set upso as to shift a detecting point of over current to a lower current sideaccording to elevation of temperature.

The current detecting circuit comprises the subsidiary switch 12composed of n-channel MOSFET, wherein the first resistance element 21 isconnected to the source terminal thereof. And the current detectingcircuit comprises the amplifier 6. A positive-terminal of the amplifier6 is connected to the drain terminal of the main switch 11, anegative-terminal thereof is connected to the source terminal of thesubsidiary switch 12 and the first resistance element 21, wherein anoutput is connected to the gate terminal of the subsidiary switch 12. Asabove described, the voltage generated in the first resistance elementand on-voltage of the main switch 11 is comparatively amplified so as tooutput it to the gate terminal of the subsidiary switch 12. Further thecurrent detecting circuit comprises the second resistance element 22that is connected to the drain terminal of the subsidiary switch 12 andconstructed so as to generate voltage by amplifying on-current of themain switch 11.

And the load circuit of this embodiment comprises a high side switch 15composed of p-channel MOSFET. A drain terminal of the high side switch15 is connected to the drain terminal of the main switch 11, thepositive-terminal of the amplifier 6 and its load, wherein a sourceterminal thereof is connected to the fixed potential 3. And a gateterminal thereof is connected to the driving circuit 4 and a CMOSinverter is made up with the main switch 11 and the high side switch 15.The switching circuit with the current detecting circuit is constructedas above-described and its fundamental operation principle is about thesame as that of the first embodiment but only polarity is inverted.

Embodiment 7

The seventh embodiment is shown in FIG. 8. This embodiment is an examplein the case that a fixed potential 3 in the fourth embodiment is anegative terminal of input voltage. In this embodiment, the firstresistance element is connected to the source terminal of a third switch13, wherein the drain terminal of the third switch is connected to thesource terminal of the subsidiary switch. And in this embodiment, thefirst resistance element has the same cell structure as that of the mainswitch 11 and uses the current detecting switch 14 constructed by MOSFETbiased by a fixed gate. And further this MOSFET has higher on-resistancevalue (10000 times for example) than on-resistance of the main switch11.

The gate terminal of the third switch 13 is connected to the drivingcircuit 4 and the gate signal of the third switch 13 is so constructedas to synchronize that of the main switch 11. That is, in thisembodiment, the main switch 11 is n-channel MOSFET so that the thirdswitch 13 is also n-channel MOSFET. In this embodiment, a delay circuit5 is connected between the gate terminal of the third switch 13 and thedriving circuit 4. The gate terminal of this current detecting switch 14is connected to the circuit 8 for generating a high-level gate voltage.

In this seventh embodiment, the gate signal of the main switch 11 andthe gate signal of the third switch 13 are constructed so as tosynchronize each other so that when the main switch 11 is in off-state,the third switch is also in off-state, and therefore when the mainswitch 11 is in off-state, over current can be prevented from flowing inthe current detecting switch 14. That can realize a current detectingcircuit with low power consumption. And since the delay circuit 5 isconnected between the gate terminal of the third switch 13 and thedriving circuit 4, delay is caused only at turn-on, so that when themain switch 11 of low-side is turned on, generation of parasiticcomponent of capacity charging current Idsp in the load circuit 2 assignal waveform can be prevented and malfunction of an over currentprotecting circuit can be prevented.

Further the first resistance element has the same cell structure as thatof the main switch 11 and has a higher on-resistance value than that ofon-resistance of the main switch 11 and the current detecting switch 14biased by the fixed gate is used, so that, the on-resistance of thiscurrent detecting switch 14 has the same temperature characteristics andvariation as these of the main switch 13, so that accuracy of currentdetection can be easily improved. Therefore a current detecting circuitwith a high reliability can be easily realized same as in case of thefourth embodiment.

Embodiment 8

The eighth embodiment is shown in FIG. 9. In this embodiment, a currentdetecting circuit with a inversed flow direction is provided, wherein itis applied in the case that the fixed potential 3 in the fifthembodiment is negative terminal of input voltage. In the currentdetecting circuit shown in the sixth embodiment and the seventhembodiment, it is assumed that current, flowing in the direction fromthe load circuit 2 through the main switch 11 to the negative terminalof input voltage, is positive. FIG. 9 shows a current detecting circuitwherein current, flowing in the direction from the negative terminal ofinput voltage through the main switch 11 to the load circuit 2, ispositive. The amplifier 6 functions the subsidiary switch 12, only whena negative voltage is charged between source and drain of the mainswitch 11. At that time, an inversed current waveform of the main switch11 is generated in the second resistance element 22 and an inversedcurrent can be detected by using a voltage waveform of the secondresistance element 22.

INDUSTRIAL APPLICABILITY

The present invention provides the effect that a current detectingcircuit, which can output large current waveform while keepingdifference of input voltage of an amplifier about zero volt, can beobtained and further provides the effect that by connecting the thirdswitch, of which gate signal synchronizes that of a main switch, betweenthe first resistance element and the subsidiary switch, voltage waveformof the second resistance element become similar to that of current waveof the main switch and that the current waveform can be thereby treatedwith high speed in shape of large voltage signal.

By means that the first resistance element made of poly-siliconresistance is formed on insulating film, insulation between the mainswitch and the first resistance element can be formed easily and ahighly integrated circuit may be possible and parasitic capacity etc.become so small that high speed operation become possible. By means thatthe first resistance element is so formed that its temperaturecoefficient has higher positive value than that of the main switch, thecurrent detecting circuit using the first resistance element can provideeasily a switching circuit with stronger function of anti-heat-bursting.Therefore, a current detecting circuit with many excellent features canbe easily formed. Therefore the present invention provides an effectthat a current detecting circuit with many excellent features can beeasily formed.

1. A switching circuit having a switching element, comprising: a currentdetecting circuit having a main switch composed of MOSFET having anon-resistance at on-voltage, wherein a gate terminal thereof isconnected to a driving circuit, and further one of a drain terminal anda source terminal thereof is connected to a fixed potential and theother terminal is connected to a load circuit; a first resistanceelement having higher resistance value than the on-resistance of themain switch; a subsidiary switch composed of MOSFET, the source terminalof which is connected to the first resistance element, an amplifier thatamplifies comparatively a first resistance element voltage generated inthe first resistance element and the on-voltage of the main switch andgenerates an output to a subsidiary switch gate terminal of thesubsidiary switch; a second resistance element connected to a drainterminal of the subsidiary switch, in which a second resistance elementvoltage is generated according to an on-current of the main switch; anda third switch, connected between the first resistance element and thesubsidiary switch in which a third switch gate terminal of the thirdswitch is connected to the driving circuit so that a gate signal of thethird switch is synchronized with that of the main switch.
 2. Theswitching circuit according to claim 1, wherein one of terminals of thefirst resistance element is further connected to the fixed potential asin the main switch.
 3. The switching circuit according to claim 1,wherein one of terminals of the first resistance element is connected toa terminal which is connected to the load circuit of the main switch,and the other terminal of the first resistance element is connected tothe source terminal of the subsidiary switch.
 4. The switching circuitaccording to claim 1, wherein the first resistance element has the samecell structure as the main switch and is MOSFET biased by a fixed gateor poly-silicon resistance.
 5. The switching circuit according to claim1, wherein the first resistance element has the same cell structure asthe main switch and is MOSFET biased by a fixed gate or poly-siliconresistance, wherein one of terminals of the first resistance element isconnected to the fixed potential as in the main switch.
 6. The switchingcircuit according to claim 1, the first resistance element has the samecell structure as the main switch and is MOSFET biased by a fixed gateor poly-silicon resistance wherein one of terminals of this firstresistance element is connected to a terminal which is connected to theload circuit of the main switch and the other terminal of the firstresistance element is connected to the source terminal of the subsidiaryswitch.